odb: move kept cache into `struct packfile_store`

odb: move kept cache into `struct packfile_store`

The object database tracks a cache of "kept" packfiles, which is used by
git-pack-objects(1) to handle cruft objects. With the introduction of
the `struct packfile_store` we have a better place to host this cache
though.

Move the cache accordingly.

This moves the last bit of packfile-related state from the object
database into the packfile store. Adapt the comment for the `packfiles`
pointer in `struct object_database` to reflect this.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>